Sustainable synthesis of magnetic petroleum coke/nonanyl chitosan composite for efficient removal of o-nitrophenol

Worldwide industrialization has grown at a rapid pace, contaminating water resources, particularly with phenolic pollutants that pose a risk to aquatic systems and human health. The goal of this study is to create an inexpensive magnetic composite that can effectively remove nitrophenol (o-NP) using adsorptive means. In this instance, a nonanyl chitosan (N-Cs) derivative was synthesized and then combined with activated petroleum coke (AP-coke) and magnetic Fe3O4 to boost its adsorbability towards o-NP and to facilitate its separation. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffractometer (XRD), Vibrating sample magnetometer (VSM), X-ray photoelectron spectroscopy (XPS), and zeta potential were employed to characterize the magnetic composite. The experimental results indicated that the Fe3O4/AP-coke/N-Cs composite possesses a greater affinity toward o-NP with a maximal efficiency reached 88% compared to 22.8, 31.2, and 45.8% for Fe3O4, AP-coke and N-Cs, respectively. The equilibrium adsorption data coincided with the Langmuir, Freundlich, and Temkin isotherm models, with a maximum adsorption capacity of 291.55 mg/g at pH 6, whereas the pseudo second order kinetic model offered the best fit to the experimental data. Besides, the developed adsorbent preserved satisfactory adsorption characteristics after reuse for five successive cycles. The proposed adsorption mechanism involves the H-bonding, π-π interaction, hydrophobic interactions and electron donor-acceptor interactions. These findings hypothesize that the constructed magnetic composite could efficiently remove nitrophenols from polluted water with high performance and ease-separation.


Activation of petroleum coke (AP-coke)
The activation of petroleum coke (P-coke) was achieved by co-activation process 38 .Accurate 10 g of P-coke was added to nitric acid solution (10.7 N) and stirred for 24 h at room temperature.Next, the mixture was filtered and the resultant powder was subsequently transferred into a crucible and subjected to physical activation by leaving it in a muffle furnace at 650 ºC for 3 h.The activated P-cock was taken out and, cooled in a dry atmosphere free of moisture and washed a couple of times before drying at 80ºC overnight.

Preparation of nonanyl chitosan Schiff base (N-Cs)
Nonanyl chitosan Schiff base derivative was prepared based on the authors previous work 39 .In brief, chitosan was dissolved in 50 mL of acetic acid solution (2% w/v) for 6 h at room temperature to have final concentration of 2% w/v.An exact quantity of nonanal (1.86 mM) was dissolved in ethanol in (10 mL) and dropped leisurely in chitosan solution under incessant stirring at 50 °C for additional 6 h.The resultant product exhibited an intense yellow color as a result of nonanyl chitosan Schiff base (N-Cs) formation.Afterwards, N-Cs solution was precipitated using sodium hydroxide (5% w/v), accompanied with filtration.The produced N-Cs Schiff base was washed with a mixture of distilled water and ethanol and left overnight for drying in a vacuum oven at 60 °C.

Preparation of Fe 3 O 4 /AP-coke/N-Cs magnetic composite
A precise quantity of the synthetized N-Cs Schiff base derivative (1.5 g) was dissolved in 50 mL of demineralized water/ethanol (2.5:1) and 5 % (w/w) aqueous solution of acetic acid and stirred for 3 h until the mixture became homogenous.Then, 1.5 g of AP-coke were added to the N-Cs solution and left under stirring at room temperature for 30 min.Subsequently, 10 mL of NaOH solution (5 mmol/L) was added drop-by-drop to precipitate AP-coke/N-Cs mixture 40 .Hence, NaOH, as a strong base, is able to increase the pH of the mixture and can consequently react with the acidic deprotonated functional groups present in chitosan, such as amino and hydroxyl groups.This pH-driven change in solubility can trigger the precipitation of the AP-coke/N-Cs composite.Later, 0.5 g of AP-coke/N-Cs composite was dispersed in 23 mL of demineralized water and afterward heated to 70 °C.An amount of 0.58 g FeCl 3 .6H 2 O and 0.3 g FeSO 4 .7H 2 O were dissolved in 40 mL of demineralized H 2 O and followed with addition to the composite mixture.The mixture was stirred for 30 min and then, 30 mL of ammonia was added dropwise to obtain Fe 3 O 4 /AP-coke/N-Cs composite.The formed magnetic composite was washed with demineralized water and then overnight dried in an oven at 100°C.A schematic diagram describes the synthesis of Fe 3 O 4 /AP-coke/N-Cs magnetic composite is presented in Scheme 1.

Adsorbent characterization
The chemical structure of the Fe 3 O 4 /AP-coke/N-Cs composite was characterized by Fourier Transform Infrared Spectrometer (FTIR-Shimadzu8400S).The morphological properties were examined by a Scanning Electron Microscopy (SEM-S4800, Hitachi, Japan).X-ray Diffractometer (XRD-MAC Science M03XHF) was employed to investigate the crystallinity of the developed composite, while the magnetic properties were examined by Vibrating sample magnetometer (VSM), and Zeta potential analyzer (ZP-Malvern, UK).Further, the elemental-surface composition was inspected by X-ray Photoelectron Spectroscopy (XPS, Axis Ultra DLD, Shimadzu, Japan).

Batch adsorption studies
The www.nature.com/scientificreports/ The adsorption temperature was changed at the temperature range of 25-45 °C.The adsorption process was performed in a shaking water bath at a constant speed of 100 rpm.After an adsorption time, an o-NP sample was withdrawn, and then the o-NP concentration was analyzed using a UV-vis spectrophotometer at λ max of 345 nm.The removal efficiency (R %) and adsorption capacity (q) of o-NP onto Fe 3 O 4 /AP-coke/N-Cs were estimated from Eqs. (1) and (2) 41 .
where, C 0 and C t symbolize the initial o-NP concentration and its concentration at an interval time, respectively.m and V represent the quantity of used adsorbent composite and the phenol solution volume, respectively.

Results and discussion
Characterization  www.nature.com/scientificreports/ to the wagging vibration of CH 2 manifested at 1376 cm -1 , while the band at 2922 cm -1 is attributed to the CH 3 vibration.In addition, the existing band at 1151 cm -1 corresponds to C-H out of the plane deformation 42 .The appeared bands at 1471 and 3375 cm -1 are ascribed to N-H, and the band at 1640 cm -1 is assigned to C=C.Thus, the FTIR spectrum of N-Cs could confirm the functionalization of Cs by nonyl group 43,44 .The AP-coke spectrum elucidated bands at 1043 and 3413 cm -1 , which are accompanied by ether and hydroxyl, subsequently.In addition to the bands at 1615 and 2925 cm -1 related to COO and aliphatic C-H, respectively 45 .The Fe 3 O 4 spectrum illustrated its distinguishing band at 576 cm -1 , and the bands at 626 and 1402 cm -1 corresponded to Fe-O stretching vibration.The bands at 1630 and 820 cm -1 belong to OH bending and OH stretching vibration band manifested at 3305 cm -112 .Finally, the spectrum Fe 3 O 4 /AP-coke/N-Cs could infer the homogeneity between Fe 3 O 4 , N-Cs, and AP-coke, where their distinct bands appear with lower intensity compared to the authentic components.

XRD patterns
The crystallographic patterns of N-Cs, AP-coke, Fe 3 O 4 , and Fe 3 O 4 /AP-coke/N-Cs composite are illustrated in Fig. 1B.The N-Cs pattern denoted the amorphous character of N-Cs with a wide band at 21.8º.Similarly, the amorphous nature of AP-coke was evinced via its crystallographic pattern, where broadband appeared at 24.5º.The Fe 3 O 4 pattern depicts the distinguishing peaks of magnetite at 30.2º, 35.7º, 43.3º, 57.04º, and 62.7º, following 220, 311, 400, 511, and 440 planes, respectively 46 .The Fe 3 O 4 /AP-coke/N-Cs pattern implied the well-fabrication of the composite since the characteristic peaks of magnetite manifested obviously.In addition, the appearance of overlapping of the AP-coke and N-Cs peaks forms a broadband centered at 24.1º.This finding was consistent with the FTIR results that suggested a good combination between Fe 3 O 4 , AP-coke, and N-Cs.

Zeta potential
The net charges magnitude on the Fe 3 O 4 /AP-coke/N-Cs composite was measured by zeta potential, as elucidated in Fig. S1.It was found that the point of zero charges was 4.67, reflecting the Zwitterionic-nature of Fe 3 O 4 / AP-coke/N-Cs around pH = 4.67.The composite has positive charges in a highly acidic medium and negative charges in alkaline media.Hence, Fe 3 O 4 /AP-coke/N-Cs could adsorb both anionic and cationic contaminants from wastewater.

XPS spectra
The elemental and chemical composition of the Fe 3 O 4 /AP-coke/N-Cs composite was investigated using XPS analysis (Fig. 2A-E).The wide-spectrum signalized that Fe 3 O 4 /AP-coke/N-Cs is composed of sulfur (S2p), nitrogen (N1s), iron (Fe2p), carbon (C1s), and oxygen (O1s).The iron spectrum clarified the presence of ferrous species, where their distinctive peaks manifested at 710.75 and 724.27 eV; in addition, the existing ferric species at 712.74 and 727.13 eV 48

Impact of adsorbent compositions
The adsorbability of the composite and its components towards o-NP was evaluated at the same adsorption parameters  www.nature.com/scientificreports/including H-bonding, π-π interaction, hydrophobic interactions, and electron donner/acceptor interaction that could be provided by Fe 3 O 4 , AP-coke, and N-Cs.

Impact of contact time
The impact of contact time on the adsorption of o-NP on the surface of Fe 3 O 4 /AP-coke/N-Cs magnetic composite was illustrated in Fig. 4B.The findings demonstrated that the adsorption process was quickly enhanced over the first 60 min with a maximum adsorption capacity of 78.30 mg/g and a maximum removal (%) of 76 %.Then, the adsorption process took place in a slow rate until it reached equilibrium in 100 min, with a maximum adsorption capacity and removal (%) values of 89.2 mg/g and 88.07 %, respectively.This phenomenon can be explained by an increase in the number of o-NP molecules that diffuse throughout the film fluid around the sufficient free adsorption sites of the adsorbent surface during the initial adsorption stage 49 .Nevertheless, there was no noticeable effect on the adsorption performance with increasing the contact time beyond 100 min, since most of adsorption sites were saturated by o-NP molecules.

Impact of pH
The most important variable influencing the adsorption process is unquestionably pH.Thus, as shown in Fig. 4C, its impact on the adsorption of o-NP by Fe 3 O 4 /AP-coke/N-Cs magnetic composite was investigated at a pH range of 2-12.At pH 6, the o-NP adsorption onto Fe 3 O 4 /AP-coke/N-Cs composite demonstrates an excellent adsorption performance, as demonstrated by the highest adsorption capacities of 89.2 mg/g and 88.07 %, respectively.This finding may be explained by the fact that o-NP has a pKa of 7.23, indicating that it exists in the molecule form at acidic circumstances 6,50 .Therefore, certain chemical and physical interactions, such as the π-π interaction and H-bonding, play a more important role in the adsorption mechanism of o-NP than the electrostatic interaction.On the other hand, as pH rose beyond 6, there was a noticeable decline in adsorption performance with increasing pH to 12 as recorded by the lowest values of 27.4 mg/g and 26.05 % for both the adsorption capacity and the removal (%) of o-NP.The strong repulsion forces between the negatively charged magnetic composite and the anionic o-NP at higher pHs may be responsible for those results.

Impact of o-NP concentration
Figure 5A shows the impact of the initial concentration of o-NP on the adsorption capacity and removal (%) values.The results refereed that the adsorption capacity meaningfully increased from 89.2 to 254.6 mg/g with increasing the o-NP concentration from 50 to 200 mg/L.Indeed, the increase in the initial concentration of pollutant reinforces its driving force that overwhelms the mass transfer struggle and enhances the migration from the bulk solution to the surface of the adsorbent.Nevertheless, the removal (%) value was declined from 88.1 to 52.8 % as the concentration of o-NP solution increased from 50 to 200 mg/L, which most probable due to the increase in the driving force reduction of the free adsorption sites, in which the entire surface sites on the surface of Fe 3 O 4 /AP-coke/N-Cs had saturated using a certain o-NP concentration.

Impact of adsorbent dose
Figure 5B illustrates how the adsorption performance of o-NP is affected by an increase in the dose of the adsorbent.As anticipated, the removal percentage of o-NP increases from 67.4 to 98.2 % with the increase in the adsorbent dose from 0.005 to 0.02 g.The observed result may be attributed to the overabundance of adsorption active sites on the Fe 3 O 4 /AP-coke/N-Cs magnetic composite surface, which increases with increasing dose and subsequently enhances the removal percentage (%) of o-NP.On contrary, as the dose was increased, the adsorption capacity value decreased from 132 to 55.7 mg/g.This may be explained by increasing the number of unsaturated active sites with raising the Fe 3 O 4 /AP-coke/N-Cs dosage at a constant o-NP concentration 51,52 .

Impact of temperature
As seen in Figure 5C, the impact of adsorption medium temperature on the adsorption process was examined in the 25-45 °C range.The findings showed that when the temperature was raised between the range of 25-45 °C, the adsorption capacity and the removal percentage both marginally rose, going from 89.2 mg/g and 88% to 99 mg/g and 97.2%, respectively.These results could be explained by increasing the segmental motion of the adsorbent composite with rising temperature.This would increase the pace at which o-NP disperses over the adsorbent's exterior boundary layer, boosting the adsorption profile.

Kinetic study
To recognize the responsible mechanism pathway for the o-NP adsorption onto Fe 3 O 4 /AP-coke/N-Cs composite, linear kinetic equations like Elovich, Pseudo first order, and Pseudo second order (Table S1) were applied in analyzing the experimental results 53,54 .Figure 6A 55,56 .The Langmuir model defined that the maximal adsorption capacity of o-NP was 291.55 mg/g.Freundlich model assured the favorability of Fe 3 O 4 /AP-coke/N-Cs to adsorb o-NP molecules in which the n value exceeded two.The Temkin model (Fig. 6F) implied the controlling of physisorption since the b value was lower than 80 kJ/mol 57 .

The plausible adsorption mechanism
The FTIR spectrum of the o-NP-loaded Fe 3 O 4 /AP-coke/N-Cs composite (Fig. S4) revealed new bands at 1407 and 1543 cm -1 , which correspond to NO 2 stretching 58 .In addition, the observed band at 867 cm -1 attributes to the benzene ring 59 .Furthermore, by comparing the wide-scan spectra of Fe 3 O 4 /AP-coke/N-Cs before and after the o-NP adsorption (Fig. 7A), it was noticed an increase in the intensity of nitrogen peak.These findings evinced the occurrence of the o-NP adsorption onto the Fe 3 O 4 /AP-coke/N-Cs surface.In addition, the oxygen  Therefore, the electrostatic interaction could not contribute to the adsorption of o-NP in acidic and neutral media.Electrostatic repulsion negatively impacts the aptitude of the anionic o-NP molecules onto the negatively charged binding species of Fe 3 O 4 /AP-coke/N-Cs in the alkaline medium.The H-bonds could be formed between the nitrogen and oxygen of o-NP molecules and the plentiful hydrogen onto the Fe 3 O 4 /AP-coke/N-Cs surface, and vice versa.Notably, nonyl-functionalization strengthens the H-bonds between Fe 3 O 4 /AP-coke/N-Cs and o-NP molecules since it provides abundant hydrogen atoms onto the composite surface.In addition, the presence of a nonyl group onto Fe 3 O 4 /AP-coke/N-Cs could enhance the hydrophobic interactions between Fe 3 O 4 /AP-coke/N-Cs and o-NP molecules.The π-π interaction is an influence mechanism pathway that could participate in the o-NP adsorption onto Fe 3 O 4 /AP-coke/N-Cs via their aromatic rings.H-bonding, π-π interaction, and hydrophobic interactions were confirmed by the peaks shifting of the carbon spectrum of the used Fe 3 O 4 /AP-coke/N-Cs composite (Fig. 7C).
The electron donner/acceptor interactions between (i) electron donner groups of Fe 3 O 4 /AP-coke/N-Cs; sulfonic, hydroxyl, and benzene ring and nitro group of o-NP (electron acceptor group), and (ii) the electron acceptor groups of Fe 3 O 4 /AP-coke/N-Cs (carboxyl) and electron donner groups of o-NP; hydroxyl and benzene ring.Moreover, the iron species of the composite could attach to o-NP molecules by forming coordination bonds.These suggestions were inferred by the peaks shifting of the sulfur, oxygen, iron, and nitrogen spectra after the o-NP adsorption (Figs.7D,E and S3).
In one word, several mechanism pathways could contribute to the adsorption process of o-NP onto Fe 3 O 4 / AP-coke/N-Cs; (i) chemical pathways such as H-bonding, π-π interaction, electron donner/acceptor interaction and coordination bonds, and (ii) physical pathways like hydrophobic interaction.These suggestions are consistent with kinetic and isotherm results that implied the participation of chemical and physical pathways in the o-NP adsorption.

Comparison with other adsorbents
A comparison study was executed between the fabricated magnetic composite with other reported adsorbents was presented in Table 3. Remarkably, the developed Fe 3 O 4 /AP-coke/N-Cs composite exposed ultimate adsorption aptitude towards o-NP compared to other adsorbents.From the comparison study, the developed Fe 3 O 4 / AP-coke/N-Cs composite recorded the highest adsorption capacity value of 291.55 mg/g which was accomplished in a shortest equilibrium time (100min).These results suggest the potential applicability for adsorptive removal of o-NP.

Removal of o-NP from actual wastewater
Actual wastewater samples were collected from the industrial drain of a pigment factory in Alexandria, Egypt.The specifications of the collected actual wastewater before and after the treatment process are listed in Table 4.The o-NP concentration in the actual wastewater sample was about 57.23 mg/L.Surprisingly, the removal efficacy of the o-NP in the actual wastewater by Fe 3 O 4 /AP-coke/N-Cs was 82.45 % after 100 min, clarifying the high efficiency and applicability of the as-fabricated composite in the actual wastewater remediation.

Conclusion
This study reported the formulation of Fe Scheme 1.A schematic pathway for the synthesis of Fe 3 O 4 /AP-coke/N-Cs magnetic composite.
FTIR spectraThe FTIR bands of N-Cs, AP-coke, Fe 3 O 4 , and Fe 3 O 4 /AP-coke/N-Cs composite are exhibited in Fig. 1A.The N-Cs spectrum revealed the characteristic peaks of C-H wagging at wavenumbers 663 and 895 cm -1 .The bands at 1033, 1258, and 1420 cm -1 are allocated to C-O-C, N-C, and C-OH bonds, respectively.The belonging bands (1) q
Figure 1C represents the hysteresis loops of Fe 3 O 4 and Fe 3 O 4 /AP-coke/N-Cs composite.The magnetism measurements clarified the superparamagnetic property of Fe 3 O 4 in which the coercivity was 8.61 G.While the coercivity of Fe 3 O 4 /AP-coke/N-Cs composite increased to 31.73 G, reflecting the ferromagnetic character of the composite.Furthermore, the saturation magnetization of Fe 3 O 4 and Fe 3 O 4 /AP-coke/N-Cs composite were 57.12 and 16.95 emu/g, implying a decline in the magnetism of Fe 3 O 4 after blinding with AP-coke and N-Cs.This decline in the Fe 3 O 4 magnetism is due to the non-magnetic performance of AP-coke and N-Cs and the low proportion of Fe 3 O 4 in the composite 47 .
. The carbon spectrum illustrated the corresponding peaks to C-O, C-C, and COO at 286.62, 284.96, and 288.53 eV, respectively.The oxygen spectrum depicted the peaks at 530.3 and 531.44 eV, which are allocated to Fe-O and C-O, respectively.The sulfur spectrum denoted the belonging peak to S-C at 164.25 eV and S-O at 166.49 eV.The nitrogen-spectrum (Fig.S2) clarified the N-H peak at 400.03 eV.Consequently, the XPS spectra denoted the well-combination between Fe 3 O 4 , AP-coke, and N-Cs since the belonging peaks to their chemical composition appeared.SEMThe SEM images of N-Cs, AP-coke, Fe 3 O 4 , and Fe 3 O 4 /AP-coke/N-Cs composite were exhibited in Fig.3A-D.By analyzing N-Cs under the SEM, it was observed that it has a sponge-like morphology with a wrinkled and rough outer surface.The specifications of the N-Cs surface enable it to carry the particles of Fe 3 O 4 and AP-coke.The AP-coke morphology looks like irregular rocky shapes with quite uneven sizes.The Fe 3 O 4 image elucidated that its particles are almost spheroidal, with a tiny size on a nano-scale.The Fe 3 O 4 /AP-coke/N-Cs image implied spreading the Fe 3 O 4 and AP-coke particles onto the N-Cs surface, assuring the feature of the N-Cs surface to act as a supporter.

Figure 4 .
Figure 4. (A) Comparison test between capabilities of Fe 3 O 4 , AP-coke, N-Cs, and Fe 3 O 4 /AP-coke/N-Cs composite towards the adsorption of o-NP, (B) Effect of contact time, and (C) Effect of pH medium on the o-NP adsorption aptitude.

Figure 5 .
Figure 5. Impact of (A) Initial concentration of o-NP, (B) Adsorbent dose, (C) and Adsorption temperature on the adsorption of o-NP onto Fe 3 O 4 /AP-coke/N-Cs magnetic composite.
www.nature.com/scientificreports/RegenerationstudyRecycling test was performed to evince the regeneration ability of the as-synthesized Fe 3 O 4 /AP-coke/N-Cs magnetic composite.The test of regeneration proceeded by adding 0.01 g of Fe 3 O 4 /AP-coke/N-Cs in 20 mL of o-NP and the adsorption process was kept under stirring for 100 min.Then, the Fe 3 O 4 /AP-coke/N-Cs composite was collected and washed by 1 M NaOH followed by distilled water.This former procedure was repeated for five o-NP adsorption/desorption cycles.Surprisingly, the recycling test results (Fig.8) implied that the removal (%) of o-NP declined by 8.93 % after the 5th cycle, reflecting the excellent regeneration ability of Fe 3 O 4 /AP-coke/N-Cs.
3 O 4 /AP-coke/N-Cs magnetic composite for adsorptive removal of o-NP.Factors affecting the adsorption process were investigated through a series of batch adsorption studies.High adsorption performance towards o-NP was accomplished by Fe 3 O 4 /AP-coke/N-Cs composite at pH 6 compared to its individual components.A sequence of adsorption isotherm and kinetic studies concluded that the adsorption of o-NP onto the composite surface was fitted to Langmuir, Freundlich, and Temkin isotherm models with a maximum adsorption capacity of 291.55 mg/g at 25 ºC and followed the pseudo second order kinetic model.Moreover, the gained XPS results assumed that the adsorption mechanism comprises H-bonding, π-π interaction, hydrophobic interactions and electron donor-acceptor interactions.Overall, the proposed magnetic composite's outstanding adsorption performance, better reusability and simple separation characteristic point to its potential application for the adsorptive removal of phenolic contaminants from aquatic systems.https://doi.org/10.1038/s41598-024-64117-1

Table 1 .
Pseudo first order, pseudo second order, and Elovich models parameters for adsorption of o-NP ion onto Fe 3 O 4 /AP-coke/N-Cs composite.

Table 2 .
Parameters of Langmuir, Freundlich, and Temkin isotherms for the o-NP adsorption onto Fe 3 O 4 / AP-coke/N-Cs composite.

Table 3 .
Comparison of maximum adsorption capacity and equilibrium contact time of Fe 3 O 4 /AP-coke/N-Cs composite with other adsorbents for the o-NP removal.

Table 4 .
The specification of the actual wastewater before and after treatment.